The present disclosure relates to an objective lens, an optical pickup, and an optical drive device.
As optical recording media for recording and reproduction signals using light irradiation, so-called optical discs such as, for example, a CD (Compact Disc), a DVD (Digital Versatile Disc), a BD (Blu-ray Disc: registered trademark), and the like have become spread.
The present applicant has proposed a so-called bulk recording type optical recording medium as disclosed in Japanese Unexamined Patent Application Publication No. 2008-135144 or Japanese Unexamined Patent Application Publication No. 2008-176902 with regard to optical recording media which lead the next generation of optical recording media which are widespread at present such as the CDs, the DVDs, the BDs, and the like.
Here, the bulk recording is a technique in which, for example, as shown in FIG. 10, laser light irradiation is performed for an optical recording medium (a bulk type recording medium 100) having at least a cover layer 101 and a bulk layer (recording layer) 102 while sequentially changing focal positions and thus multi-layer recording is performed inside the bulk layer 102, thereby achieving a large recording capacity.
For such bulk recording, a recording technique called a micro hologram type is disclosed in Japanese Unexamined Patent Application Publication No. 2008-135144.
In the micro hologram type, a so-called hologram recording material is used as a recording material of the bulk layer 102. As the hologram recording material, for example, light cured photopolymer is widely used.
The micro hologram type is largely classified into a positive micro hologram type and a negative micro hologram type.
The positive micro hologram type is a method in which two light beams opposite to each other are collected at the same position so as to form fine interference fringes (holograms), which are used as recording marks.
In addition, the negative micro hologram type is a method in which, in contrast to the positive micro hologram type, interference fringes which are formed in advance are erased by laser light irradiation, and the erased portions are used as recording marks. Specifically, in the negative micro hologram type, an initialization process for forming interference fringes on the bulk layer 102 in advance is performed before a recording operation is performed. The initialization process is performed by irradiating the bulk layer 102 with two light beams by parallel light to be opposite to each other, and forming interference fringes on the overall bulk layer 102. During recording, in a state of focusing on an arbitrary layer position of the bulk layer 102 on which the interference fringes are formed as described above, information is recorded using the erasure marks by performing laser light irradiation according to recording information.
Further, the present applicant has proposed a recording method of forming voids (vacancies) as recording marks, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2008-176902, as a method of the bulk recording different from the micro hologram type.
The void recording method is a method in which laser light irradiation is performed for the bulk layer 102 made of a recording material such as, for example, light cured photopolymer at relatively high power, thereby forming voids (vacancies) inside the bulk layer 102. As disclosed in Japanese Unexamined Patent Application Publication No. 2008-176902, the vacancy portions formed in this way have a refractive index different from other portions in the bulk layer 102, and thus reflectance of light at the interfaces can be heightened. Therefore, the vacancy portions function as recording marks, and thereby information recording is realized by the formation of the vacancy marks.
Since the void recording type does not form holograms, recording may be completed through light irradiation from one side. In other words, it is not necessary to collect two light beams at the same position and form recording marks unlike the positive micro hologram type.
Upon comparison with the negative micro hologram type, there is an advantage in that the initialization process is not necessary.
In addition, although an example where when void recording is performed, pre-cure light is applied before the recording is described in Japanese Unexamined Patent Application Publication No. 2008-176902, void recording can be performed even if the application of the pre-cure light is omitted.
However, a recording layer (a bulk layer) of a bulk recording type (hereinafter, simply referred to as a bulk type) optical disc recording medium where the above-described variety of recording methods are proposed does not have an explicit multi-layer structure in the meaning that, for example, a plurality of reflection layers are formed. That is to say, the bulk layer 102 is not provided with a reflection layer and a guide groove for each recording layer which a typical multi-layer disc has.
Therefore, in a state of the structure of the bulk type recording medium 100 shown in FIG. 10 described above, a focus servo or a tracking servo may not be performed during the recording where the marks are not formed.
For this reason, in practice, the bulk type recording medium 100 is provided with a reflection surface (reference surface) which has guided grooves as shown in FIG. 11 and is used as a reference.
Specifically, guide grooves (position guiders) by, for example, formation of pits or grooves are formed at the lower surface side of the cover layer 101 in a spiral shape or a concentric shape, and a selective reflection layer 103 is formed thereon. In addition, the bulk layer 102 is laminated on the lower layer side of the cover layer 101 where the selective reflection layer 103 is formed in this way, via an adhesive material such as, for example, a UV cured resin as an intermediate layer 104 in the figure.
Here, absolute position information (address information) such as, for example, radius position information or rotation angle information is recorded through the formation of the guide grooves by the pits or grooves as described above. In the following description, a face (in this case, the face on which the selective reflection layer 103 is formed) where the guide grooves are formed and absolute position information is recorded is referred to as a “reference face Ref”. In addition, the “reference face Ref” may be referred to as a “servo signal face”.
After the above-described medium structure is formed, as shown in FIG. 12, the bulk type recording medium 100 is irradiated with servo laser light (also simply referred to as servo light) as laser light for position control independently from laser light for recording marks (or reproduction of marks) (hereinafter, also simply referred to as recording and reproduction laser light or recording and reproduction light).
As shown in the figure, the recording and reproduction laser light and the servo laser light are applied to the bulk type recording medium 100 via a common objective lens.
At this time, if the servo laser light reaches the bulk layer 102, there is concern that the servo laser light has an adverse effect on the mark recording in the bulk layer 102. For this reason, in the bulk recording type in the related art, laser light having a wavelength band different from that of the recording and reproduction laser light is used as the servo laser light, and, as a reflection layer formed on the reference face Ref, the selective reflection layer 103 having wavelength selectivity of reflecting servo laser light and transmitting recording and reproduction laser light therethrough is provided.
Based on the above-described premise, an operation of when marks are formed on the bulk type recording medium 100 will be described with reference to FIG. 12.
First, when multi-layer recording is performed for the bulk layer 102 where the guide grooves or the reflection layer is not formed, on which layer position marks are recorded in the depth direction of the bulk layer 102 is set in advance. In the figure, a case is exemplified in which a total of five information recording layer positions L, first information recording layer position L1 to fifth information recording layer position L5 are set as layer positions on which marks are formed (mark forming layer positions: also referred to as information recording layer positions) in the bulk layer 102. As shown in the figure, the first information recording layer position L1 is the information recording layer position L set at the uppermost part, and, thereafter, the information recording layer positions L2, L3, L4 and L5 are sequentially the information recording layer positions L set at the lower layer side.
Here, the information recording layer positions L may also be represented as “information recording depth”.
In addition, here, for convenience of illustration, although a case where the number of the information recording layer positions L is five is exemplified, in practice, the number of the information recording layer positions L is expected to be, for example, on the order of several tens.
Here, during recording where the marks are not formed, a focus servo or a tracking servo may not be performed for each layer position in the bulk layer 102 based on reflection light of the recording and reproduction laser light. Therefore, a focus servo control and a tracking servo control of the objective lens during the recording are performed such that a spot position of the servo laser light tracks the guided grooves on the reference face Ref based on reflection light of the servo laser light.
However, it is necessary for the recording and reproduction laser light to reach the bulk layer 102 formed at the lower layer side of the reference face Ref for the mark recording, and further a focus position in the bulk layer 102 can be selected. For this reason, an optical system in this case is provided with a focus mechanism (independent recording and reproduction light focus mechanism) for independently adjusting a focus state of the recording and reproduction laser light separately from the focus mechanism of the objective lens.
Here, an outline of an optical system for performing recording and reproduction of the bulk type recording medium 100 including the recording and reproduction light focus mechanism is shown in FIG. 13.
As shown in FIG. 13, an objective lens also shown in FIG. 12 can be displaced in the radius direction of the bulk type recording medium 100 (tracking direction) and the direction coming into contact with and separating from the bulk type recording medium 100 (focus direction) by a biaxial actuator.
In FIG. 13, the independent recording and reproduction light focus mechanism in this case is a mechanism of a type called an expander, and, as shown, includes a fixed lens, and a movable lens which is held so as to be displaced in a direction parallel to the optical axis of the recording and reproduction laser light by a lens driving unit. In the independent recording and reproduction light focus mechanism, the lens driving unit drives the movable lens such that collimation of the recording and reproduction laser light incident to the objective lens in the figure varies, and thereby a focal position of the recording and reproduction laser light is adjusted independently from the servo laser light.
Further, since, as described above, the recording and reproduction laser light and the servo laser light have wavelength bands different from each other, reflection light beams of the recording and reproduction laser light and the servo laser light from the bulk type recording medium 100 are separated toward respective systems (that is, the respective reflection light beams can be detected independently from each other) by a dichroic prism of the figure in the optical system in this case so as to corresponding thereto.
In a case of light on the outgoing path, the dichroic prism has a function of synthesizing the recording and reproduction laser light and the servo laser light on the same axis and enabling the synthesized light to be incident to the objective lens. Specifically, in this case, as shown in the figure, the recording and reproduction laser light is reflected by a mirror via the independent recording and reproduction light focus mechanism, is reflected by a selective reflection surface of the dichroic prism, and then is incident to the objective lens. On the other hand, the servo laser light passes through the selective reflection surface of the dichroic prism and then is incident to the objective lens.
FIG. 14 is a diagram illustrating a servo control during reproduction of the bulk type recording medium 100.
When the bulk type recording medium 100 on which marks have been recorded is reproduced, it is not necessary to control a position of the objective lens based on reflection light of the servo laser light unlike the recording. In other words, during the reproduction, it is preferable to perform a focus servo control and a tracking servo control of the objective lens based on reflection light of the recording and reproduction laser light by targeting a mark string formed on the information recording layer positions L (also referred to as information recording layers L or mark forming layers L during the reproduction) to be reproduced.
As described above, in the bulk recording type, the recording and reproduction laser light for recording and reproduction the marks and the servo light as position control light are applied to the bulk type recording medium 100 through the common objective lens (through the synthesis on the same optical axis), and, during the recording, the focus servo control and the tracking servo control are performed such that the servo laser light tracks the position guiders on the reference face Ref. In addition, the mark recording can be performed at necessary positions (the depth direction and the tracking direction) inside the bulk layer 102 by separately adjusting a focus state of the recording and reproduction laser light using a spherical aberration correction mechanism even if guide grooves are not formed in the bulk layer 102.
Further, during the reproduction, the focus servo control and the tracking servo control of the objective lens are performed based on reflection light of the recording and reproduction laser light such that a focal position of the recording and reproduction laser light tracks the mark string which has been recorded, and thereby it is possible to reproduce the marks recorded in the bulk layer 102.
However, according to the structure of the bulk type recording medium 100 shown in FIG. 11, the bulk layer 102 is provided via the cover layer 101 having the necessary thickness, and thus it is necessary to correct spherical aberration when marks are recorded and reproduced at the respective layer positions L in the bulk layer 102. That is to say, in this case, since the cover thickness is different for each layer position L, the correction of the spherical aberration is performed such that the correction amount is different for each layer position L.
In the optical system shown in FIG. 13, the independent recording and reproduction light focus mechanism performs the correction of spherical aberration along with the adjustment of a focal position. Specifically, the correction of spherical aberration is performed by making an offset (defocus) of a focal position of the recording and reproduction laser light from the layer position L which is a target of recording and reproduction by a predetermined amount.
Here, as such, the drive device of the bulk type recording medium 100 performs the spherical aberration correction according to the difference in the cover thicknesses (that is, the difference in the information recording layer positions L); however, which layer position L is used as a reference of the spherical aberration correction is important when the spherical aberration correction is performed for each layer position L.
For example, if an optical system is designed such that the spherical aberration for the information recording layer position L1 which is the uppermost layer is minimized, the correction amount of the spherical aberration become excessive when recording targeting the information recording layer position L5 is performed. Particularly, in the bulk recording type, as the thickness of the bulk layer 102, for example, about 200 μm to 300 μm is now under review in order to achieve a large recording capacity through the multiple layers, and, in this case, it is necessary to perform a large spherical aberration correction so as to correspond to the thickness of 300 μm to the maximum.
Therefore, the optical system is designed such that the spherical aberration is minimized at the layer position L which is intermediate in the bulk layer 102. According thereto, the maximal correction amount of the spherical aberration necessary when recording and reproduction are performed in the same range of the information recording layer positions L1 to L5 can be suppressed to a half of a case of designing the optical system using the information recording layer position L1 as a reference.
Hereinafter, as such, the layer position L (layer position L minimizing the spherical aberration) used as a reference in the design of the optical system when the spherical aberration correction is performed is referred to as a reference layer position.
As can be seen from the above description, the drive device of the bulk type recording medium 100 is configured so as to correct the spherical aberration occurring due to an error between an layer position L which is a target of recording and reproduction and the reference layer position (in this case, L3).
Although description using illustration is omitted, on the other hand, an actual drive device is configured to perform a tilt correction by tilting the objective lens (so-called lens tilt) so as to handle the disc tilt.
Specifically, as a tilt detection, for example, a tilt of the bulk type recording medium 100 is detected, and, a tilt mechanism which tilts the objective lens is driven according to the detection result, thereby performing the tilt correction (comatic aberration correction).